1. Field of the Invention
The present invention relates to a driving force transmission system to be applied to a transmission, transfer or differential of a vehicle.
2. Related Art
Generally in the driving force transmission system of the vehicle, there are arranged a plurality of rotary members, through which the output of an engine is transmitted to wheels. When a clutch mechanism is arranged in the driving force transmission system between the rotary members, on the other hand, the transmission/interruption of a torque can be arbitrarily switched by applying/releasing the clutch mechanism, if necessary. One example of the driving force transmission system, in which the clutch mechanism is arranged in the torque transmission path of the rotary members, is disclosed in Japanese Patent Laid-Open No. 282019/1991 (JP-A-3-282019).
The driving force transmission system, as disclosed, is equipped with a differential carrier made hollow, a connecting shaft (or a first rotary member) inserted in the differential carrier, and a hub (or a second rotary member) arranged in the differential carrier and made rotatable relative to the connecting shaft. The connecting shaft and the hub are arranged on an axis, and a bearing is fitted between the differential carrier and the connecting shaft.
On the differential carrier, on the other hand, there is fitted an annular electromagnet which is stopped in its rotation relative to the differential carrier by means of bolts. On the connecting shaft, moreover, there is fixed a cylindrical side wall to set a predetermined air gap between the side wall and the electromagnet. A drum is jointed to the outer circumference of the side wall, and a pilot clutch and a main clutch are arranged between the drum and the hub.
The pilot clutch is equipped with a clutch disc splined in the inner circumference of the drum, a cam member fitted axially movably on the outer circumference of the hub, a clutch plate splined in the outer circumference of the cam member, and an armature to be attracted toward the side wall by an electromagnetic force. On the other hand, the main clutch is equipped with a clutch disc splined in the inner circumference of the drum, and a clutch plate splined in the flange of the hub.
Between the cam member and the main clutch, moreover, there is arranged an axially movable push member. Between the cam member and the push member, still moreover, here are fitted balls. On the other hand, a drive pinion shaft is jointed to the hub, and the drive pinion shaft is jointed to the differential constructed by the gear mechanism.
According to the driving force transmission system thus constructed, when no electric current is fed to the electromagnet, the pilot clutch is released so that the torque of the connecting shaft is not transmitted to the hub. When the electric current is fed to the electromagnet, on the other hand, the magnetic flux passes through the side wall and the armature so that the armature is attracted toward the side wall by the electromagnetic force.
Then, the pilot clutch is applied to rotate the cam member, and this rotation is transmitted to the balls so that the push member is activated in the axial direction to apply the main clutch thereby to transmit the torque of the connecting shaft to the hub. The torque thus transmitted to the hub is transmitted through the drive pinion shaft, the differential and axles to the wheels.
On the other hand, the differential carrier is filled in its internal space with oil for lubricating and cooling the pilot clutch, the main clutch and the differential.
Here, in the pilot clutch or the main clutch, the torque is transmitted by the frictional force between the surfaces of the clutch disc and the clutch plate. It is, therefore, preferable to select the oil which can keep the wear resistance, the oil cutting property or the judder resistance of the clutch disc and the clutch plate satisfactory. In the differential, on the other hand, the torque is transmitted and differentiated by the meshing resistance of the gears. It is, therefore, preferable to select the oil which is low in the fluid point, and is excellent in the heat resistance and oxidation stability and load resistance.
In the driving force transmission system described in the aforementioned publication, however, the pilot clutch, the main clutch and the differential are arranged in a common space of the differential carrier. This makes it difficult to employ the oil which is suited individually for the pilot clutch, the main clutch and the differential. As a result, the active characteristics of the pilot clutch and the main clutch may be degraded to lower the transmission function of the driving force.
Moreover, the oil, as confined in the differential carrier, may wet the electromagnet to make the magnetic permeability of a magnetic circuit heterogeneous and to lower the transmission function of the clutch mechanism for the driving force. Still moreover, a foreign substance such as wear powder, as produced at the side of the differential, may enter the clearance between the clutch disc and the clutch plate to wear or break the clutch mechanism thereby to lower the durability or the transmission function of the driving force.